R1:e6fb1dc1e9dc

When an install is instanced, include the instance identifier in the URI for file data

Summary:
This allows us to CDN the cluster.

General problem is that we can't easily give each instance its own CDN URI (`giraffe.phcdn.net`) in Cloudfront, because it requires that you enumerate all aliases (and there's a limit of 100) and depends on SNI (a newish feature of SSL which allows one server to serve multiple certificates, but which doesn't have full support everywhere yet).

It's //possible// that we could…